1. Field of the Invention
The present invention relates to a video signal processing circuit for use in a video tape recorder (VTR), television set, and the like.
2. Description of the Prior Art
Recently comb filters have become widely used in signal processing circuits treating video signals.
In the case of the NTSC signal, for example, the spectra of the chrominance signal (hereinafter to be briefly called C signal) are interleaved between those of the luminance signal at intervals of the horizontal scanning period (the period to be briefly called 1 H). The luminance signal (to be briefly called Y signal) is separated from the input video signal by a Y type comb filter which adds a 1 H delayed video signal to the input video signal, and the chrominance signal is separated from the input video signal by a C type comb filter which subtracts the 1 H delayed video signal from the input video signal. Thus, the comb filters are used in the so-called Y/C separating circuit. The C type comb filter is used also in the color S/N improving circuit for the VTR.
These comb filters, however, function on condition that the vertical correlation of the chrominance signal is large. During the period in which the vertical correlation is small, the Y/C separation does not function well and such bad results are thereby produced as color fringing and dot interference. There have been proposed various methods for detecting presence or absence of the vertical correlation of the chrominance signal. One method detects the vertical correlation of the Y signal utilizing the fact that the correlation between the Y signal and the C signal is large, but this causes malfunction with a signal in which there is no correlation between the Y signal and the C signal.
Another method is proposed by Yves C. Faroudja in Japanese Laid-open patent application No. 56-40382 for detecting the correlation when the signal level of the chrominance signal frequency component and the signal level of the luminance signal frequency component are both larger than a predetermined level. A simplified block diagram of this prior art method is shown in FIG. 13. According to this construction, when a chrominance signal as shown by a in FIG. 14 is inputted, a signal b as a detected output by the C type comb filter and a signal c as a detected output by the Y type comb filter are passed through an AND gate 81, and a signal as indicated by d is thereby outputted.
When a signal which includes a difference only in the phase as shown by a in FIG. 15 is inputted, however, a signal as indicated by d will be outputted even if the signals b and c are passed through the AND gate, so that the correlation of the chrominance signal will not be detected (this is a disadvantage with this method). In other words, with the arrangement as described above, it is impossible to detect the absence of the vertical correlation of the chrominance signal from a signal which includes neither correlation between the C signal and the Y signal nor change of amplitude in the vertical direction of the chrominance signal but only includes a change of hue. Therefore, it is unavoidable that the trouble like color fringing or dot interference is produced.